1. Field of the Invention
The present invention relates to a line driver, more particularly to a line driver that is capable of automatically adjusting an output impedance.
2. Description of the Related Art
A transmission line is used to transmit high-speed signals, and a line driver is used to drive the input to the transmission line with a higher than normal signal level. However, in order to reduce signal reflection, an output impedance of the line driver must be matched with a characteristic impedance of the transmission line.
FIG. 1 shows a conventional active terminal line driver which forms a part of an integrated circuit (not shown). The line driver is coupled to a load 18 that has a resistance (R1). The line driver is used to amplify an input current (Iin) to produce an output voltage for driving the load 18. The line driver comprises a differential amplifier 11, a pair of series variable resistors 12, 13, a pair of negative-feedback resistors 14, 15, and a pair of positive-feedback resistors 16, 17. Each of the series variable resistors 12, 13 includes a plurality of resistor elements (not shown), and a plurality of switches (not shown) that are controlled to vary coupling states of the resistor elements and thereby change a resistance of the respective series variable resistor 12, 13.
Referring to FIG. 2, an equivalent half circuit of the line driver may be constructed to determine an output impedance (Zout) and a transimpedance (1/Gm) thereof as follows:
            Z      out        =                            V          out                          I          out                    =                                    R            s                                1            -                                          R                f                                            R                p                                      +                                          R                s                                            R                p                                                    ≅                                            R              s                                      1              -                                                R                  f                                                  R                  p                                                              ⁢                                          ⁢                      (                                          when                ⁢                                                                  ⁢                                  R                  p                                            >>                              R                s                                      )                                          1              G        m              =                            V          out                          I          in                    =                                    R            f                                1            +                                          R                s                                                              R                  l                                //                                  R                  p                                                      -                                          R                f                                            R                p                                                    ≅                                            R              f                                      1              +                                                R                  s                                                  R                  l                                            -                                                R                  f                                                  R                  p                                                              ⁢                                          ⁢                      (                                          when                ⁢                                                                  ⁢                                  R                  p                                            >>                              R                1                                      )                              
where (Vout) is an amplitude of the output voltage, (Rs) is a resistance of each of the series variable resistors 12, 13, (Rf) is a resistance of each of the negative-feedback resistors 14, 15, (Rp) is a resistance of each of the positive-feedback resistors 16, 17, and (R1) is the resistance of the load 18.
Therefore, the amplitude (Vout) of the output voltage is as follows:
      V    out    =            I      in        ×                            R          f                          1          +                                    R              s                                      R              l                                -                                    R              f                                      R              p                                          .      
Due to manufacturing variations in the integrated circuit (variation ratios of the resistors 12-17 are substantially the same, that is, ±20%), the output impedance (Zout) and the transimpedance (1/Gm) will be varied correspondingly, and the output impedance (Zout) is approximately directly proportional to the resistance (Rs) of each of the series variable resistors 12, 13.
The conventional technique of correcting the output impedance (Zout) so that it corresponds to its design value involves adjusting the resistance (Rs) of each of the series variable resistors 12, 13. Also, in order to fix the amplitude (Vout) of the output voltage, the conventional technique involves generating the input current (Iin) (the input current Iin may be affected by manufacturing variations) with reference to an internal resistor (not shown) of the integrated circuit so as to counterbalance a variation in the transimpedance (1/Gm).
Since correction using conventional techniques entails utilizing equations of the output impedance (Zout) and the amplitude (Vout) of the output voltage after having undergone simplification, the output impedance (Zout) and the amplitude (Vout) of the output voltage will still be varied from their intended values.
In addition, since the resistance (Rs) of each of the series variable resistors 12, 13 is extremely small (typically a few tens of ohms), the switches for the series variable resistors 12, 13 must be very large so as to reduce conduction resistances of the same. More space is required to implement such large switches. Also, conduction resistances of the switches may be influenced by the output voltage and thereby reduce the linearity of the output voltage.